More evident roles of nNOS for the regulation of Ca2+‐sensitivity and SERCA activity in the right than the left ventricular cardiomyocytes of rats

Abstract

It is known that neuronal NOS (nNOS, NOS1) regulates the intracellular Ca2+ ([Ca²⁺]i) and contractility of cardiomyocytes, which was investigated only in the left ventricular myocytes (LVCM) while not in the right ventricle (RVCM). In the paced cardiomyocytes, we have recently reported more relaxed diastolic sarcomere length (SLD) and smaller shortening (∆SL) while the amplitude of Ca²⁺ transient (∆[Ca²⁺]i) were not different (Jeon et al., 2021). Here we compared the expression and the functional roles of nNOS in the RVCMs and LVCMs from Sprague‐Dawley rats. The total expression and the phosphorylated Ser1417 ratio of nNOS were not different between RVCM and LVCM. However, the treatment with nNOS‐specific inhibitor, S‐methyl‐L‐thiocitrulline (SMTC, 100 nM), induced more prominent augmentation of ∆SL along with faster contraction while paradoxical faster relaxation and decreased ∆[Ca²⁺]i in RVCM. The hysteresis curve between ∆SL and ∆[Ca²⁺]i­ showed more prominent leftward shift, i.e. sensitization of the myofilaments in RVCM by SMTC. Consistently, the speed of contraction became faster, and the longer SLD of RVCM became similar to that of RVCM by the treatment with SMTC. Intriguingly, the SMTC treatment accelerated the Ca2+ removal by SERCA in RVCM, which appears to be consistent with the decreased ∆[Ca²⁺]i and faster relaxation. As for the decreased myofilament calcium sensitivity through sGC‐cGMP‐PKG pathwy, altered phosphorylation of troponin I (TnI) was suggested. In fact, the SMTC treatment decreased the TnI phosphorylation in RVCMs, while not LVCMs. Immunoblotting revealed higher expression of nNOSβ and its phosphorylated form in the myofilament fraction from RVCM than LVCM. In summary, the nNOS‐dependent longer SLD and the paradoxical negative inotropic effect despite the inhibitory effect on SERCA might be mediated by the myofilament‐associated nNOSβ decreasing Ca2+‐sensitivity through TnI phosphorylation. Interestingly, we also found that the treatment with omecamtiv mecarbil, a recently proposed cardiac myosin activator, induced more prominent inotropic effect in RVCM, which might imply a larger functional reservoir for the RV contraction.